The technical field of this invention is vehicle traction control.
Motor vehicle traction control systems using both wheel brake and propulsion throttle control are provided on many vehicles, since they can share components and control algorithms with anti-lock brake controls that are used on the majority of vehicles. A typical known traction control system is responsive to wheel speed sensors on the driven and undriven wheels of the vehicle to detect excessive wheel spin (rotational speed) in a driven wheel and apply the brake to that wheel to reduce wheel speed to a target wheel speed that is greater than an undriven wheel speed by a calibrated target delta velocity. The target delta wheel velocity is calibrated to produce an improved traction between the tire and the road surface. But braking of an over-spinning wheel alone is not always an optimal traction control strategy on a smooth but low coefficient road surface, due to the extra heat and wear that may be produced. Thus, the systems supplement selected driven wheel braking with a throttle reduction command provided to the propulsion unit of the vehicle to reduce power to the driven wheels.
A rough road surface provides a challenge to such systems in that the road may actually have a high surface coefficient of friction; rather it is the roughness of the surface causing intermittent reduction in normal force of the wheel on the road surface that produces an intermittent loss of traction. Although the brakes can react sufficiently quickly to deal with such fast changing traction conditions, it is undesirable in many cases to attempt to force the vehicle engine to vary its power at such a high rate.
The invention is a method and apparatus for controlling the traction of a motor vehicle in which the control emphasis is changed in response to detection of a rough road surface so as to shift traction control of a specific wheel away from a reduction in propulsion power toward braking control of the wheel.
The method and apparatus of the invention control the traction of a driven wheel included in a plurality of wheels supporting a vehicle on a road surface with a propulsion unit supplying power to the driven wheel and a brake braking the driven wheel. Brake pressure to the driven wheel is modulated to maintain the speed of the driven wheel at a first target wheel speed greater than an undriven wheel speed of the vehicle by a calibrated delta target wheel speed value. A road roughness signal indicating a level of roughness of the road surface is provided. When the road roughness signal indicates a first level of road roughness, a first level of power reduction is provided to the driven wheel from the vehicle propulsion unit. But when the road roughness signal indicates a second level of road roughness greater than the first level of road roughness, a second level of power reduction less than the first level of power reduction is provided to the driven wheel from the propulsion unit.
The rough road signal is preferably derived from vertical body/wheel motion responsive sensors at one or more of the driven wheels and undriven wheels but may alternatively be derived from one or more of the wheel speed sensors. The method and apparatus of the invention preferably applies a greater calibrated target delta wheel speed value at the second level of road roughness than at the first level of road roughness. The method and apparatus of the control may further modulate brake pressure to the driven wheel in response to a time derivative of a difference between a rotational wheel acceleration of the driven wheel and an undriven wheel rotational acceleration, the time derivative having a derivative gain, with a first gain value when the road roughness signal indicates third level of road roughness and a second gain value lower in magnitude than the first gain value when the road roughness signal indicates a fourth level of road roughness greater than the third level of road roughness. Further objects and advantages of the invention will be apparent from the accompanying drawings and following description of a preferred embodiment.